Balllistics Program Question

[Blaine Fields]

Let's say that on the day you decide to zero your scope, the following conditions exist: 300 ft. altitude, 48 deg.(F), 30.10 in.Hg., 65% humidity. You zero at 100 yds.

Two weeks later, you take a shot at a target 1000 yds. away. On that day, you're at 2500 ft., 65 deg. (F), 29.54 in.Hg., 74% hum. You happen to have a PDA with the Horus ballistics software and punch in the atmospheric data, crank in the elevation indicated and take the shot. You are way high.

The problem is that the program (and every other external ballistics program I've seen) assumes that you've zeroed under the current conditions and the scope data is from that current zero. The program doesn't know that you've zeroed under different atmospheric conditions. What is needed, therefore, is a an offset to move the zero to current conditions. It seems to me that this offset will apply uniformly to the new data. In other words, let's say that a 100 yd. shot under current conditions with the earlier zero would wind-up with a hit .75 MOA high at 100 yds. It seems to me that all current data would have to be adjusted by subtracting this offset from all elevation data in order to correct for the zero obtained under different conditions.

So, first: is this right?

Second: if it is, what is the easiest way to compute this offset? (I need a formula here.)

 

[Guest]

Blaine

(A-B)/c/1.047 = MOA correction from old zero to new target at current altitude

A = Actual bullet drop from borline at new Range and Altitude in inches

B = Actual bullet drop from boreline at 100 yd. old zero at old Altitude

C = range in yards divided by 100

Just a shot in the dark, but let me know how it works, never actually tried it.

[ 01-07-2003: Message edited by: S1 ]

[ 01-07-2003: Message edited by: S1 ]

 

[Dave King]

Blaine

Atmospherics at 100 yards are probably close to a non-issue. (There seems to be something else at work at 100 yards when changing geographic locations though, Len and I have discussed this 100 yard zero problem before.)

In tests I performed in Colorado (change from 600 ASL to 10,500 ASL) I didn't see much difference in trajectories until about 400 yards. (Shot with 600 ASL data at 10,500 ASL just to check elevation change.)

In the TRGT Data Book there is a Zero Data Summary page that allow a shooter to track data at many ranges and temperatures. A fella could easily make one for other atmospheric and geographic conditions.


S1

I don't believe your supplied formula is correct as written. There seems to be a method to remove the 100 yard zero data from the "old" conditions but no place to reintroduce the corrected "new" 100 yard data.

 

[Blaine Fields]

Dave,

I think you're right. From S1's post, I decided to look at bullet drop which is simply a function of time. For a 178 gr. in 308 at 2675 at the muzzle, total bullet drop for the 100 yd. trip was 2.55 inches. Elevating the temp and lowering the pressure significantly lowered the drop to 2.54 inches, an insignificant amount. So it would appear that once zeroed at 100 yds., any change of atmospherics would have no real effect on that zero.

Of course, if the initial zero was at 600 yds., a correction factor would be required and using bullet drop over time would seem to be the approach to computing this factor.

Thanks.

 

[CAM]

I don't know if this is the same, I thought all the BC's were from Standard Conditions, Sealevel-59 Deg.-BP 29.53-RH 78%. Then changing conditions basicly changed your BC?

Also do velosity change with conditions (temp sensitive powder)and unless you shoot a crony how to know without actual testing?

All ballistic programs need MV is the MV the same at sealevel as at 10,000 ft? How do you guys know what MV to use with out actual testing at different elev? say you have 3450 FPS at 5000 ft Elev. and you plan to hunt at 10,000 ft elev

What form or info do you record at the test range? ( temp, Vel, RH, BP, sunny, etc. )
Is there a good form to print out for all the info?

 

[Brent]

Also do velosity change with conditions (temp sensitive powder)and unless you shoot a crony how to know without actual testing?

You don't...

Recording the actual velocity drop at different temps and adjusting the MV input on the program is the only way to be sure and accuraate. This will affect the drop more than the density change temp variations cause, in most cases unless the powder used is very temp insensitive.

 

[Dave King]

Cam

Temperature sensitive powders are a bit of a problem, try to stay away from the worst or operate with that understanding.

Elevation and other atmospherics (excluding temperature sensitive powders) will have little or no effect on Muzzle Velocity (MV), it will effect the downrange trajectory by increasing or decreasing the flight time for a given (similar) distance.

I use the same MV at any altitude, I do verify long range trajectory prior to a shoot at different elevations (significant elevation changes). I have a ballistic prediction before entering the new area and verifiy the data then make a comparison with the "home" (in my case, flat land) data.

I print my own "zero" targets in Microsoft Word. Use the grid feature and set the grid to 1", add a header and footer with the data you feel is important. I include, distance, date, time, temp, wind speed, powded name and weight, bullet name and weight, rifle, cartridge (including brass name and prep), primer, trigger weight, scope name and setting. I have a wind and light "circles". On the side I have numbered lines for the load (shot) number and MV as well as my "call" info. I use these targets in paired sets, one goes downrange to the backer and the other stays on the firing line with the rifle. I plot and number the impact position of the rounds as they are fired, I later use this with the MV to find a "sweet spot".

Standard printer paper is a little fragile and the holes sometimes rip, I use clear packing tape on the back of the downrange target to make them more durable (a needed additive).

 

[Guest]

Dave

It is much simpler than your trying to make it. If you know the actual bullet drop at your new range and relative air density, relative to the bore line, All you have to do is subtract your angular adjustment from the boreline that is already dialed on the scope under the 'guise' of being your old 100 yd. zero, at the old relative air density. What you get is the MOA move needed to hit the new target at the new relative air density, from the position of the original zero. I thought that was the original problem Blaine wanted to solve? If you want to get more exact, we need to throw in the Sight height stuff and your just about perfect.

[ 01-07-2003: Message edited by: S1 ]

[ 01-07-2003: Message edited by: S1 ]

 

[Dave King]

S1

Help me out here, I'm apparently stuck on an incorrect assumption.

"(A-B)/c/1.047 = MOA correction from old zero to new target at current altitude

A = Actual bullet drop from borline at new Range and Altitude in inches

B = Actual bullet drop from boreline at 100 yd. old zero at old Altitude

C = range in yards divided by 100 "


Two scenarios:

1000 yards at 10500 ft ASL

200 yards at 10500 ft ASL

1000 yards first:

(A) Drop from boreline to 1000 yards = 347.1 inches

(B) Drop from boreline at 100 yards = 2.5 inches

(C) Range in yards divided by 100 (1,000/100) = 10

(347.1 - 2.5) = 344.6

344.6 / 10 = 34.46

34.46 / 1.047 = 32.913... (new 1000 yard come-up)

Now the 200 yards:

(A) Drop from boreline to 200 yards = 10.2 inches

(B) Drop from boreline at 100 yards = 2.5 inches

(C) Range in yards divided by 100 (200/100) = 2

(10.2 - 2.5) = 7.7

7.7 / 2 = 3.85

3.85 / 1.047 = 3.677... (new 200 yard come-up)


Now compare the "corrected" come-up to the original (non converted) come-up.

10500 ft ASL 1000 yard pre-converted = 347.1/10/1.047= 33.151...

10500 ft ASL 200 yard pre-converted = 10.2/2/1.047= 4.87...

The necessary MOA change for the 1000 yard shot is (33.151 - 32.913) = .238. I'll overshoot by .238 MOA (or about 2.5 inches)using the non-converted come-up.

The 200 yards MOA change is (4.87 - 3.67) = 1.2. I'll overshoot by 1.2 MOA (or about 2.5 inches) if I use the non-converted come-up.



It appears to me that all I'm removing is the first 100 yards flight time drop of 2.5 inches in either case. I know that I don't need 1.2 MOA correction at 200 yards at 10500 ASL and then only .23 MOA correction at 1000 yards.... What am I missing?

 

[Guest]

Dave
Your not missing anything, the formula is missing is the sight height correction, we need the actual sight height above the bore line to finish.

 

[*WyoWhisper*]

Holy Cow.. my head is starting to hurt.....

 

[Darryl Cassel]

Hello All

This is just another reason we use the "spotter Shot" or two method "FIRST" when working over 1000 yards. NO drop chart is 100% correct when the conditions change from day to day after you have made the chart.

The spotter shot and a good spotter with bigeyes will eliminate the negatives of trajectory to the live animal.

DC

 

[TiroFijo]

Let´s have an aspirin together

...
If you are firing a 308 with a 175 SMK bullet @ 2680 fps, standard conditions and zero altitude, drop at 100yds is 2.53", and let's say sight height is 1.6" (this example is with the Sierra Infinity program, but exact numbers are not important).
In order for line of sight to coincide with the bullet trajectory at 100yds, you have to slope down your line of sight (or the erector tube inside the scope) to compensate for the sight height and the bullet drop at that distance. This compensation is 2.53+1.6 = 4.13" in 100yds.
Using a little trigonometry, we find that the proyection of the bore line crosses the line of sight at 38.74yds, and this means that the bore is pointing high 4.13*9.6126 = 39.7" at 1000yds.
If you substract this value from your drop at 1000yds (424.1") you have the bullet path relative to the line of sight at this point: 424.1-39.7 = 384.4". So, our sight correction for 1000yds would be 36.7 MOA.

Corrections to bullet drop at 100yds for air temperature, pressure and altitude are too small (this means the bullet keeps the same 100yds zero), but this same round fired at 20ºF could easily be going 100 fps slower (2580 fps), so the bullet drop is now 2.72". Using the above calculations we see this causes a 2.72*2.53 = 0.19" variation in our 100yds zero, and a 0.19*9.61 = 1.83" variation at 1000yds, this is a 0.17 MOA error at 1000yds, IMHO this is "lost in the noise", even with this extreme environmental change.

Edited for correctness...

[ 01-08-2003: Message edited by: TiroFijo ]

 

[CAM]

Brent and Dave

Thanks for the answers and the data points.

The reason I ask about MV is because its affects on down range look to be more than most other Items.

MV is the result of internal ballistics and is the most important start for external ballistics.

I have heard guys keeping there bullets in a cooler to control powder temp and Elevation should not matter untill external ballistics.

I will do some averaging and just use one MV

I'll take two aspirin and reread the formula!

Thanks again

P.S. Dave I think you are using 100 and 1000 data when you should only compare old 100 and new 100 then you could compare old 1000 to new 1000 ? NOT SURE

[ 01-08-2003: Message edited by: Cam ]